Vlado Ličina
1, Tore Krogstad
2, Aleksandar Simić
1, Milica Fotirić Akšić
1and Mekjell Meland
31 Faculty of Agriculture, University of Belgrade, Serbia
2 Faculty of Environmental Sciences and Natural Resource Management, Norwegian University of Life
NIBIO REPORT | VOL. 7 | NO. 59 | 2021
Precision fertilization to apple trees
A review
TITLE
Nutrition and fertilizer application to apple trees - a review
AUTHOR
Vlado Ličina, Tore Krogstad, Aleksandar Simić, Milica Fotirić Akšić and Mekjell Meland
DATE: REPORT NO.: AVAILABILITY: PROJECT NO.: ARCHIVE NO.:
18.03.2021 7/59/2021 Open 11138 18/01468
ISBN: ISSN: NO. OF PAGES: NO. OF APPENDICES:
978-82-17-02809-3 2464-1162 79 2
EMPLOYER:
Norwegian Institute of Bioeconomy Research (NIBIO)
CONTACT PERSON: Mekjell Meland
KEYWORDS: FIELD OF WORK:
Apple, Malus domestica. Borkh., rootstock, nutrient reserves, storage organs, leaf analyses, soil testing, liquid fertilizer, foliar nutrition, macroelements, microelements.
Horticulture
SAMANDRAG/SUMMARY:
Denne rapporten gjev eit litteraturoversyn ved ulike sider av næringsopptaket av ulike mineral og tilførsel til epletre. Det er omtalt dei fysiologiske sidene av næringsopptaket hjå grunnstammer og sjølve sorten, transporten gjennom sil- og vedvev og funksjonen til dei ulike minerala i epletreet.
Bladanalysar er eit viktig diagnoseverktøy for vurdering av næringsstatusen i treet. Terskelverdiar for dei ulike minerala er vurderte og tiltak for å retta opp eventuelle mangelsymptom ved hjelp av eit bladgjødslingsprogram. Tilsvarande er eigenskapar ved jorda vurderte, normer for jordanalysar omtalte og tilråding om kalking. Siste bolken omhandlar ulike måtar og mengder for å tilføra gjødsel til frukthagen. Dette gjeld gjødsling til jorda, gjødselvatning i dropevatningsystemet i trerekkja og bladgjødsling.
Literature data is reviewed for physiological aspects of nutrients adsorption by apple tree rootstocks, their storage in perennial scion parts and their mobilisation during the vegetative growth. Nutrients functions and needs are also presented and discussed. Although it is well known that several nutrients can influence fruit quality and disorder of apple and their efficiency, accumulation and storage in trees organs become a scope of many investigations. Therefore, the common tool used in fruits improvement concerns adequate use of soil fertilizers, foliar fertilizers, and finally, fertigation, as measured regularly used in apple growing practice.
COUNTRY: Norway
COUNTY: Vestland
MUNICIPALITY:
SITE: Lofthus
APPROVED
Inger Martinussen
NAVN/NAME
PROJECT LEADER
Mekjell Meland NAVN/NAME
Preface
The aim of this project is to increase the total production of Norwegian produced apples of high quality produced in an environmentally friendly way. The total fruit consumption in Norway is increasing.
Nevertheless, the Norwegian apple production is stable during the last ten years with some variations between the years. The R&D partners Norwegian Institute of Bioeconomy Research (NIBIO
Ullensvang), Norwegian University of Life Science (NMBU) and Faculty of Agriculture, University of Belgrade, Serbia in cooperation with the national fruit advising services will study and develop the fertilization recommendations of the Norwegian apple tree orchards. The main task was to study the relationship between main plant physiological principles in the tree, relate it to the soil, tree growth, yield, fruit quality, and fruit storage. The international literature about fertilization of apple trees is studied which is compiled in this report.
Project owner is Hardanger Fjordfrukt in cooperation with the partners Nå Fruktlager, Sognefrukt, Innvik Fruktlager and their growers.
NIBIO Ullensvang was R&D responsible by Mekjell Meland
This project ‘Presisjonsgjødsling til epletre’ was funded by The Research Council of Norway (Forskningsmidlene for jordbruk og matindustri) - project No. 281968.
Lofthus, 17.03.2021 Mekjell Meland
Content
1 Introduction ... 6
1.1 Trends in apple production ... 7
2 Apple rootstock properties... 8
2.1 Effect of rootstock on scion features ... 8
3 Apple rootstocks supply ... 11
3.1 Nutrient adsorption ‐ Transport mechanism ... 11
3.2 Rhizosphere effect ... 12
3.3 Root anatomy‐radial and longitudinal gradient of root ... 12
3.4 Vascular system in perennial woody parts of the apple tree ... 14
3.5 Xylem sap ... 14
3.6 Phloem loading ... 15
4 Functions of minerals in apple nutrition ... 18
5 Foliar analyses – Leaf diagnosis ... 22
5.1 Leaf tissue analysis ‐ Critical level of nutrients in apple leaves ... 23
5.2 How to “read” and how to use data from leaf mineral analyses ... 25
6 Foliar nutrition and its efficiency in apple production ... 28
7 Apple orchard location, soil physical and chemical properties ... 34
7.1 Climatic conditions ... 34
7.2 Soil depth of apple orchard soils ... 35
7.3 Water content in apple orchard soils ... 35
7.4 Soil texture in apple orchards’ soils ... 36
7.5 Chemical properties of apple orchard soil ... 37
7.6 pH of apple orchard soils ... 37
7.7 Cation exchange capacity in apple orchard soils ... 43
7.8 Soil organic matter in apple orchard soils ... 43
8 Introductions to the nutrient’s availability in apple’s orchard soil ... 46
9 Nutrient management of apple orchards ... 49
9.1 Fertigation in apple production ... 49
9.2 Fertilization program for young apple trees ... 51
9.3 Fertilization program for bearing apple trees ... 54
9.4 Nutrient application before and during the vegetative season in apple orchards ... 56
9.5 Nutrient application by foliar nutrition ... 62
9.6 Fertigation of apple orchards ... 63
10 Discussion about the results of the soil testing obtained from two laboratories ... 66
References ... 70
Vedlegg ... 79
1 Introduction
Apple represents the 3rd most important fruit crop worldwide (comes immediately after watermelons and bananas) and it’s the most important deciduous fruit tree. As a fresh fruit, it is consumed daily throughout the year. In addition, apple fruits can be used to the various forms of processing (juice, food pastes, jellies, jams, concentrate, apple cider, cheese syrup etc.). According to FAOSTAT (2017), world apple production is 89,329,179 tons with increasing tendency from the past few decades. The largest producer is China, representing around 48% of the world’s total production. The USA is the second largest producer with 6.1% followed by Poland (3.8%), India (2.9%), Turkey (2.9%) and Italy (2.9%). The apple production started long time ago, around 4000 BC. Central Asia, Himalayan regions of India, Pakistan including western China is believed as centre of apple origin (Muzher et al., 2007).
Nowadays, over 63 countries produce apple with a great variability of growing conditions, by utilizing about 7500 of mostly local cultivars which usually serves as row material for breeding or variety selection (Dobrzañski et al., 2006).
In the past few decades, world apple production is characterized by intensified growing technology with more than 2.000 trees per hectare, high yield per tree and/or unit area, introduction of new cultivars and dwarfing rootstocks and satisfactory fruit quality. However, apple growing requires demanding technology, making this culture one of the most difficult fruit species to cultivate.
systems/technologies during the past period. Demanding fruit hand picking has been partly solved or compensated by cultivation of small trees, which has been achieved by the use of low-vigour growing rootstocks selected for spur types of apple cultivars. In the meantime, climate changes and increased disease pressure underline an urgent need for new, highly productive apple cultivars that are resistant to biotic and abiotic stresses. Besides the selection of cultivars adapted to the climatic stresses which are common in apple worldwide production, the selection and breeding of new resistant cultivars to various pathogens nowadays are dominant, facilitating their application in apple production.
Therefore, apple breeding and selection has been done more intensively than any other fruit in the world. As a result of spontaneous and planned hybridization, over 10.000 noble apple cultivars are developed and constantly new and better one is being developed.
One can imagine that apple fruits with large size, good coloured peel, shapely and marked aroma which has a high market value are the highest demanding task for breeders. However, this fruit contains numerous bioactive primary and secondary compounds with potentially valuable nutritive, health and pharmacological potential (Lv, 2016, Kviklys et al., 2014, 2017). All this nutritive, health and pharmacological beneficial of apple fruit has been concisely given in an old Welsh proverb that most of us are familiar with: “An apple a day keeps the doctor away”. That is to say that apples are widely consumed as a rich source of phytochemicals, while epidemiological studies have linked the consumption of apples with reduced risk of some cancers, cardiovascular disease, asthma, and diabetes (Manach et al., 2004). In the laboratory, apples have been found to have very strong antioxidant activity, inhibit cancer cell proliferation, decrease lipid oxidation, and lower cholesterol.
Apples also contain a variety of phytochemicals, including quercetin, catechin, phloridzin and
chlorogenic acid, all of which are strong antioxidants. The phytochemical composition of apples varies greatly between different varieties of apples, and there are also small changes in phytochemicals during the maturation and ripening of the fruit (Boyer and Liu., 2004). Recent studies have shown that the cultivar may substantially influence the fruit chemical properties, especially phenolic content and total antioxidant activity reflecting a qualitative genetic control (Drogoudi et al., 2008). Therefore, apple breeding/selection has diverse tasks related to better fruit quality, better pomo-technological properties, different ecological and phytosanitary constraints, and, finally, to the products with higher nutritional and health value (Nikolić and Akšić, 2009). That is specially aimed for varieties which undergo organic production, recently one of the trendiest ways of apple production. The improved
health beneficial of organically produced apples has been sufficiently documented in numerous scientific publications (Peck, 2004, Grzyb et al., 2012, Heinmaa et al., 2017).
1.1 Trends in apple production
Over the last 60 years apple production around the world was subjected to the radical modification by changing traditional production systems to the production with the use of dwarfing rootstocks and smaller tree row space, means, modern orchards now range from 1,000 up to the 6,000 trees/ha. A fertigation system is used as mandatory. Besides, high density orchards have been conducted to the renewal pruning and specific treatments with growth regulators aimed for fruit thinning, which result in high, steady yields and first-class fruit quality. The transition in apple production historically took significant time and usually lasted for years, where the final acceptance and adoption for practice have happened after full review of apple tree behaviour during the application of introduced technologies.
Principally, economic interests and labour engagement are the major driving force for efforts in developing new technological trends in apple production.
Currently applied dwarfing rootstocks are mostly results of systematic and severe work of two world famous rootstock Research Stations (East Malling Institute, Merton and John Innes Institute, both England), which basically gave the frame of rootstock traits in selection, also transferring proposed demands to the other breeders worldwide (US, Canada, UK, Sweden, Russia, Poland, Germany, the Czech Republic, Israel, Romania and Japan). As a result, it could be said that today’s huge world’s apple production is based on dwarfing rootstocks use and great number of trees per square area. In such modern high-density orchards, some other beneficial of renewed apple production has been emphasized like a large effect on tree precocity (flowering and cropping in the early years), as well as more exposed labour efficiency in high density orchards. Apple trees in those orchards also influence the partitioning of the trees resources between vegetative growths and large cropping, what is a absolutely new challenge tendency in apple scions’/rootstock botanical behave. Other characteristics that also have become important include improvement of fruit size, tolerance to diseases (fire blight, phytothphora, apple scab, apple mildew, apple replant disease and crown gall), tolerances to insects and tolerances to abiotic stresses (drought, water excess, spring frost and winter cold tolerance).
Naturally, such spur apple grafted trees include some limitations, such as a lack of winter hardiness, poor anchorage, root suckers, sensitivity to some apple disease and fragile graft unions. Some others disadvantage of minor importance could be also present at some selections of the rootstock.
2 Apple rootstock properties
The function of roots include anchorage, the absorption of water and mineral nutrients, and synthesis of various essential compounds, such as growth regulators, and usually serves as storage organ for plant’s nutrients. Also, root serves as an active bridge between live and solid phase of the soil, being a primary zone of contact with soil organisms. In perennial crops, such as apple, grafting is used to join the matching root systems (rootstocks) to shoots (scions) that produce the harvested products.
Rootstocks varieties vary substantially in architecture and function, both within and between selected rootstock species, and they are a crucial component in coordinating plant responses to a range of abiotic and biotic factors, mainly influencing the growth and their mechanical strength as an
important factor in preventing overthrow of trees by wind and winter injury. The selection of rootstock for certain apple production must be evaluated in order to choose the rootstock that shows the best characteristics for specific soil and ecological conditions.
Nowadays, one of the most important rootstock requirements is their capacity to control tree vigour that allows high-density planting. The easiest method of tree vigour control, a rootstock selection is the most important factor for high density apple orchards that produce large fruits and more fruits per hectare serving thus as a very important economical factor influencing the profitability of fruit growing (Autio et al., 2000; Tworkoski and Miller, 2007; Webster and Wertheim, 2003). On the other hand, besides the productivity, the rootstock can also affect lifetime of the orchards, therefore affecting economical aspects for plantation owners. Dwarf trees and low height can be targeted to fulfil the crop- protection sprays accurately, therefore, to avoid excessive use of pesticides and undesirable spray coast, which has negative side effect on surrounding environment.
Modern orchards are established on dwarf and semi-dwarf rootstocks. The rootstock M.9 has become the most dominant for apples for its suitability in high-density plantings. At present, more than 25 sub-clones of M.9 are bred in Europe (Kosina, 2010). Consequently, the greatest interest in tree size control is the use of rootstock that produces trees near the size of M.9 (Crassweller et al., 2001). Over the last 20 years new selections of M.9 has been made and developed in many countries. Pajam 1 and Pajam 2 were selected in France (Masseron, 1986). Rootstocks Jork 9 (Faby et al., 1986), Burmenger 984 (Baab, 1998) and Supporter 1, 2, 3, and 4 (Fischer, 1997) were promoted in Germany. A clone such as NAKB T 337 has been propagated in the Netherlands (Kosina, 2002).
2.1 Effect of rootstock on scion features
Grafting typically employs two individuals, where usually rootstock and scion, possess a desired variety feature. Clone genetic copy rootstocks and sexually produced scions are typically used during the cultivar breeding and selection process. Therefore, the root systems accomplish these two units, having a mutual impact on both grafted parts. Much is known about many scion traits in general (fruit characteristics, variety patterns, yield formation). However, the impact of rootstocks on these scion phenotypes remains unclear. For example, each apple rootstock has its own distinct characteristics, regarding such aspects as winter hardiness, anchorage, insect and disease resistance, site and soil adaptability, and also controls various aspects of the scion such as degree of dwarfing, precocity and productivity (Lauri et al., 2006, Tworkoski and Miller, 2007, Ferree et al., 2001b, Ferree et al., 2001a, Hirst and Ferree, 1995, Hirst and Ferree, 1996, Tubbs, 1974, Webster et al., 1985, Drake et al., 1988, Fallahi et al., 1985). Due to graft incompatibility, not all rootstocks are compatible with all scions, the special attention in rootsock selection has been paid to a wide range of possible scion use.
It is still not clear how rootstocks achieved their effect on scion vigour and shoot growth. This question trigs numerous investigations which can give the answer not only about “dwarfing effect” but also about the rootstock effects on all other positive physiological consequences which kept grafting as regular practice in seedling production for years (abundance of flowering, fruit set, high yields and
yield quality). Early investigation simplified these effects claiming that the lower scion loading with water and nutrients has been induce by rootstock or its imperfect graft interconnection union with scion. Also, it has been suggested that this distorted xylem settings imposes an axial resistance to water flow resulting in shoot water deficits which limited a shoot growth (Warne and Raby, 1938;
Beakbane, 1956; Tubbs, 1973). Later, all these approaches include a water hydraulic rootstock/scion imbalance made by grafting, focusing on differences existing in root part, which is lower, then in above ground apple parts (Cohen and Naor, 2002). The recent studies, however, emphasized the changes in the production and movement of plant hormones within the tree brought about using the rootstock.
The theory is that the rootstock, or possibly its graft union with the scion, alters the ratios and
concentrations of the growth-promoting hormones, such as auxins, gibberellins or cytokinins, and the inhibiting hormones, such as abscisic acid, which are translocated within the tree (Ferree and
Warrington, 2003). As a result, shortage of the stimulatory effect of indole acetic acid (IAA) in xylem development and cambial activity in woody species exist (Sundberg et al., 2000). Greater phloem differentiation rather than xylem was observed in more dwarfing rootstocks (Aloni, 1995). As an approach related to the influence of plant hormones on plant growth, a rootstock also modifies
morphology or phenology of different growth habits of scion including branch orientation. This change in shoot or branch orientation are based on a fact that more horizontal branches have less growth then vertical. All this regulation derives from apical dominance and apical control, as a tissue points where the growing substance, preferentially hormones, are transfer and focused (Miller and Tworkoski, 2003). A smaller growth and regulated apical dominance has been partly regulated by smaller auxin efflux from the rootstock. Also, a shortening of internodes, consequently, the appearance of the flower buds, could be explained as a hormonal allocation from the scion parts (Webster, 2001; Tworkoski and Miller, 2007). These above brief points are presented to emphasize the variety of evident parameters which can influence a formation of spur type of scion apple trees. Therefore, “dwarfing effect” of rootstocks is a very complex process, which include intern cumulative and interactive metabolic outcomes which operate simultaneously in both plant parts.
Besides the importance of the use of spur type in apple production, there are certain criteria common to all good clonal (vegetative propagation) rootstocks, which could be essential to seedling producers and fruit growers. Some of them are crucial. It should be said that the most important is freedom from virus and bacterial diseases. Nowadays, it is fully accepted that orchards should be planted without seedling which possess a bacterial disease crown gall (Agrobacterium tumefaciens) and collar rot (Phytophtora spp.), which induces limitations to established new plantings. Today, however, there are more serious threats to the apple orchards, since many of them are planted on dwarfing rootstocks as M.9 and M.26. They are highly susceptible to the bacterial disease fire blight (Erwinia amylovora Burill.). This bacterial infection starts during blossom time, passing through blooms, resulting leaves and branch dieback, leading in the most cases to the death of the whole tree just in a couple of days.
Accordingly, after blossom infection, bacteria can penetrate the branches, passing down toward the trunk showing no symptoms before getting to the rootstock. The susceptibility of different scion varieties was not observed, but the high susceptibility of rootstocks M.9 and M.26 can endanger not only the new plantings, but also present high density orchards (Roberts et al., 1998; Norelli et al., 2003; Russoet et al., 2007). Regardless that the apple fire blight poses as a serious threat to the whole world apple production, problem arises with the fact that disease symptoms are not very familiar to the apple growers due to that the prone diseases areas/countries are not well and systematically defined. However, it is worth to stress that this bacterial disease in a close past, literary cleaned huge arias of planted quince and pear, relatively in a very short period what was generally officially not announced.
Some other demands are also present in rootstock production and distribution. It should have an easy propagation and good bud or graft compatibility with the scion. The chosen rootstock should be capable of controlling the vigour of the scion trees to the level required by the grower, followed by the consistent and abundant cropping of large high-quality fruits. Today the rootstock
resistance/tolerance to biotic stress factors (high/low temperature, water deficiency) are specifically emphasized, which are posing as a general problem related to the climate changes. Also, if possible, the rootstocks should have the smallest production of suckers, which create a practical problem to the apple grower.
3 Apple rootstocks supply
3.1 Nutrient adsorption ‐ Transport mechanism
One can imagine that given genetic visible diversity of apples varieties as a different fruit shape, fruit taste, its dissimilar colour or a different biomass and specific tree architecture, could be applied also to the “hidden” below ground rootstock system. Such differences in properties of rootstocks are not expressed at the level of above ground parts. Besides its evident impact on developed scion vigour, rootstock differs in their ability to absorb nutrients from the soil and in the ability to transport them to the above ground portions of the trees (leaves and fruit). Another phenomenon that has been
described in the literature and perhaps discussed by producers is the effect of rootstock on fruit quality and storage. It makes sense that if rootstocks have a significant effect on nutrient concentration in fruit, they may play a role in the supplying trees with nutrients as a plant reserves aimed mostly for starting vegetation and in the case of nutrient deficiency.
The rootstock supplies the apple plant with nutrients and water. Concerning that water use is a special issue between plant species, apple rootstocks varieties/clones differs in water adsorption capacities due to their wide range of root biomass and spreading capability in soil profile. As a live mediator between solid phase and water in soil, rootstock biological system has been powered by the energy stored in different form of chemical compounds as sugars and other energy rich molecules. Generally, this energy use influences the capability in nutrient adsorption by breaking their chemical bondage.
This is the reason why the intensity of photosynthesis has a direct or indirect effect on the nutrient supply.
A basic function of taking up nutrients by the rootstock system could be compared to the mining operations. The certain nutrients require little energy and are easily available, whereas others require quite a bit more energy because they are either tightly held by the chemistry of the soil particles or because they are rare in the soil. Trees have developed several mining strategies to get what they need from different soil profiles. For example, for a relatively abundant nutrient, like potassium ions after fertilizer application, apple trees in general, allow them to be absorbed into the roots by so called
“passive transport”, with no energy use. This “passive transport” also occurred at regular calcium absorption by apple trees generated by transpiration stream. The concentration of potassium ions in the plant tissues is usually hundred or thousand times higher than in the soil solution, and as a consequence energy is needed for its adsorption. This energy consuming adsorption is called “active transport”. For the most of nutrients that may not be so readily available, apple roots may employ a combination of energy dependent mining and passive transport. This includes processes of activation of formed “ion channel” in the root membrane enabling a “passive transport”, and the use of
structural membrane constituents of “carrier proteins” and “ion pumps” in cell membrane (“active transport”). Such specific protein structural creation in the membrane structure refers to so called
“integral proteins” or “trans-membrane proteins” (TMP). They pass through whole cell membranes, and they are crucial for ion adsorption, ion selectivity and energy consumption. In general, the whole process of plant nutrition is based on the activity of these membrane constituent parts.
Such particular structure of the root membrane, made from rigid parts of cellulose microfibrils, embedded in a matrix of polysaccharides (hemicelulose and pectin substances), and bonded with specific structural protein chains, made existing specific transmembrane proteins (TMP) responsible for most of the nutrient transfer held at the root surface. Therefore, while membrane porosity mostly regulates a transfer of small molecules of water and gases (oxygen, carbon dioxide) with no electrical charges through membrane, nutrient intake is generally regulated. However, it should be always keep in mind that water makes up to 60% to 70% of the total weight of the cell wall (Hall et al., 1982), making membrane structures soaked into the water medium. The presence of water and the presence
of these active protein complexes in the form of integral, peripheral and anchored proteins at the root surface, made this plant parts very dynamic in soil media.
3.2 Rhizosphere effect
Surrounding soil, which coated a root system as a thin soil layer, is not inert. It could be said that this surrounding soil with thickness about 1-2 mm are influenced by living root and it is called rhizosphere.
This root-soil interface has been named by the German phytopathologist Lorenz Hiltner in 1904. The rhizosphere has some features which indicate the root transmembrane protein activities like an acidification of its area achieved by proton and organic acids exclusion. Namely, the living roots have this ion/proton extrusion permanently, which happens in nanoseconds, and this extrusion works looks like a firework. Consequently, this proton extrusion makes a disbalance between ion concentration and electric charge surpass/deficit in root cell (cytosol) and exterior rhizosphere space
(apoplasm/rhizosphere). As a result, a rhizosphere pH may differ from the bulk soil pH by up to two units, what could be of great importance for the pH-dependent solubility of some nutrients, especially micronutrients. However, the most important factor for root-induced changes in rhizosphere pH is the uptake of nutrients, which is coupled with proton (H+) extrusion through root membrane. This driving force for nutrient uptake by root cells is a creation an outward positive gradient in electro potential and pH between the cytosol (pH 7–7.5) and the apoplasm (pH 5–6). Transmebrane protein complex responsible for this proton exclusion is ATPase (PM-ATPse), as a protein channel which creates different electrochemical potential gradient by exclusion of charged protons. The anion uptake has been creating by proton–anion co-transport (symport) and cation uptake via proton–cation counter transport (antiport).
Dispute on the effects on pH in rhizosphere, plant roots can modify the rhizosphere chemistry in several ways: a) by release and uptake of organic compounds, b) by gas exchange (CO2/O2) related with respiration of roots and rhizosphere microorganisms, and c) by root uptake as well as release of water and nutrients (Neumann and Römheld, 2011). Roots also can modify the physical properties of the rhizosphere, such as aggregate stability, soil water capacity and numbers and size of micropores by their growth through the soil as well as release of different organic substances. In the mining process for nutrients and water, a root makes a constant pressure on solid phase which reaches up to the 6 bars, as an average of 1-3 bars (Curl and Truelove, 1986), what can be compared by the used force per squared millimetre during concrete drilling. Besides the organic compound release and the influence of growth process on organic residue root debris in rhizosphere, a root system also excludes
substances which serves as chelators or ligands which promotes a nutrient availability. Such
organically packed metal nutrients generally are disabled to react with soil solution anions, also, their capability for transport through the soil solution is much easier.
The rhizosphere effects in soil are spaced in radial and longitudinal orientation along the roots zone.
Depending on the rhizosphere processes considered (exudation of organic compounds, respiration, uptake of mobile nutrients and water) the radial extent of the secretion effect declines with increasing distance from the roots. The distance of diffused or excluded substance largely depends on soil properties and adsorption characteristics of the compounds. Adsorption depends on a type of molecular compound excluded, its molecular weight and its electrical charge, where ones with low- molecular-weight easily could be transferred to the respect distance from the root (1-2 cm), or it could be blocked by its charge on close soil particles or on root wall. Concerning that these organic
substances are food for microorganisms, proportionally its number decline from the axe of the root.
3.3 Root anatomy‐radial and longitudinal gradient of root
The capacity of nutrient uptake greatly depends on the radial gradient of root. The nutrients mobility and their solubility influence its transfer through the rhizosphere, where poorly mobile nutrients such a P, K, ammonium and micronutrients with low concentrations in the soil solution are frequently
depleted in the rhizosphere by rapid root uptake, whereas soluble nutrients, for example Ca and Mg, may accumulate close to the roots surface (Hinsinger et al., 2009).
The longitudinal gradients of roots are strongly related to its morphology. Along the roots axe are present four different root zones, changing from soft meristematic tissues up to the firm woody parts of the root branches, depending upon the degree of maturation and activities taking place in each area.
These regions, starting at the tip and moving upwards towards the stem, are the root cap, zone of active cell division, zone of cell elongation, and zone of maturation. The root cap is cup-shaped and loosely covered by a mass of parenchyma cells that covers the tip of the root. It is quite large in some plants, while in rootstocks is invisible or nearly absent. This cap presents a unique feature of roots, with structure not comparable with any part of root or stem. From its shape, structure and location, its primary function is to protect the other root cells under their abrasion and assists the root in
penetrating the soil. Replenish and rebuilt a great number of such cap cells are produced to replace those which are damaged or lost during the root mining process in soil. The movement is assisted by a slimy substance, mucigel, which is produced by cells of the root cap and epidermis. After the root growth and its work in root penetration it has been observed that this cell residue stays as a microbial food which is associated with the root live phase of this boring tip.
A mass of actively dividing cells lies behind the root cap called zone of cell division or root apical meristem. Similarly, to the other meristems, dividing the cells gives the growth and body of the root, and this is happening behind the root cap. The parenchyma cell of the meristem is small with high density, where the most of cell divisions occur along the edges of the root core. This built up the columns of cells arranged parallel to the root axis. The apical meristem of the root is organized in three primary meristems: protoderm, which gives rise to the epidermis; procambium, which produces xylem and phloem; and the ground meristem, which produces the cortex.
The region of elongation, which joins with the apical meristem, usually is not visible as a distinct part of the root. It extends about one to three centimetres or less from the tip of the root of perennial plants. At this zone, the cells grow and become several times bigger reaching somewhat of their original length or wider. Cellular expansion in this zone is responsible for pushing the root cap and apical tip forward through the soil. In the root of perennial plants, this zone is marked as a zone of calcium absorption. This could be explained as a need of plants to use of Ca to build and to support the firmness and grown wall structure. If embedded in cell wall, calcium makes bridges between glycoside units of the neighbour microfibre chains, taking over the role of the element charged for regulation of other ion membrane transfer.
In nutrient and water adsorption apical maturation root zone plays an important role. The cell character developed at this root part, which belongs to the endodermis and exodermis tissues, are relatively incomplete barrier to dissolve and attached ions and molecules, but still with the ability to control their transfer into tissues. This possess only membranes of living organisms. The presence of root hairs in this sub-apical root zones increases the absorption surface area for nutrients and water.
Their increased membrane activity is usually responsible for an increased release of protons and organic compounds into the root zone. Generally, the capabilities of a nutrient uptake and water adsorption have been attributed exclusively to the root hairs. However, nutrient adsorption is also present at the other part of roots, even much older and with more complex anatomy, which do not belong to the apical zone (Kong et al., 2014). Probably, the great influence on such nutrient adsorption plays a present microbial population, which digest organic compounds excluded or released from root cells or thrown fragments of cell walls. They make low molecular ion organic complexes in the root vicinity, suitable for ions release and later their uptake. Therefore, the statement that root hairs play a primary role in nutrient and water adsorption could be attributed only for the cereal or one-year plants, which significantly contribute to the adsorption of some low soluble and low mobile elements (phosphorus). It seems that the role of root hairs to water and mineral absorption is variable and deserves more study especially at the perennial plants.
3.4 Vascular system in perennial woody parts of the apple tree
The inner parts of wooden structure of the apple rootstocks have a cell configuration and vessel transport construction which is similar to other wooden species. It means that they possess increased lignin content as a main distinction between plant common cell wall and membrane and plant cell wall of wooden trees. This difference exists within increased lignin content and its participation up to 30%
in cell wall structure. This given firmness of the wooden plant cell wall is unique for plants. On the contrary, the wood phase in root tissues and root structure supports a hardness of this organ, while it plays a minor role in nutrient adsorption process. This process generally belongs to the “soft tissues”
as already described. In this tissue, two ways of nutrient and water transport are possible: symplast transport, transport from cell to cell via plasmodesmata (protein channels between cells), or, away from the leaving cells, via apoplast transport, means, by the use intracellular spaces. First one is very slow and used between adjacent cells, while symplast is a way of connecting cells mostly in tissue of organs, like in a root, leaves, fruits, flowers, etc.
After adsorption, water and nutrients are transported upwards. This transport takes place in the vascular system of xylem and phloem. If this transport concerns a transport of solutes from root to the shoot, it is called a long-distance transport. In woody plants, predominantly it occurred in non-living xylem vessels. Main forces which create a xylem transport are a gradient of hydrostatic pressure (root pressure) and gradient in water potential. Besides, many other factors contribute to this water and solute transport, mainly presented in literature as result of pure physical calculations an complex formulas. If we simplified this, it could be said that if pure free water has a potential of zero, the values for other solutes are usually negative and becomes less negative from atmosphere up to the root cells and soil solution. As a result, this potential discrepancy generates the upward solute movement. In days when plants transpired a great amount of water from the soil, this water potential gradient between roots and shoots grow quite steep. This is usually happening during hot days when stomata are open and plants by intensive transpiration regulate their temperature regime. So, the water in the plant can be considered a continuous hydraulic system, connecting the water in the soil with the water vapor in the atmosphere. This permanent suction power of roots makes possible a crucial supply of plants with other quantities of water and nutrients.
In contrast to the xylem, long-distance transport in the phloem takes place in the living sieve tube cells. In phloem vessels transport goes in one direction or bidirectional. Practically it means that phloem connects shoots and roots, transferring elements and synthesized organic compounds to every part of the plant. Sometimes this transfer of elements and organic solutes concerns extensive exchange processes and include internal cycling. However, these vertical connections in trees can indicate a nutritional status of above ground parts to the root. This is usually explained as a ’’signalling’’. Such substances which are sensitized in the green parts and reach a root system are largely investigated.
Between substances detected, in signalling processes are hormones, sugars, nitrogen compounds and some secondary metabolites.
3.5 Xylem sap
The chemical composition of xylem sap is greatly dependent on different factors. The presence of cations and anions mixed with different organic salutes in xylem sap are result of rootstock activities, assimilation of nutrients from soil and metabolic pathways by which they were subjected in plant organs. The concentration of salutes is also influenced by dilution by water, having therefore a close connection with the water uptake and plant’s transpiration rate. The amount of salutes greatly depends on the plant age season when the sap control is carried out. This is especially visible in apple tree in the spring, when stored reserves of nutrients and organic compounds are remobilized for the growing season. It’s also important to say that character of the xylem sap is changeable during the season, even during the time of the day of measuring.
The present metals in the xylem sap exist as separate ions but mainly in organic form complexes with organic acids. Amino acids (asparagine, aspartic acid, glutamine, glutamic acid and arginine), special peptides, small proteins and other different organic acids (malic, citric, oxalic and succinic acid) serve as a carrier of metals in plant xylem system. Therefore, their presence in xylem sap influences pH, and the reactivity of present ions. Calcium, Mg, Fe, Mn and Zn are likely to be transported in the xylem as cations or cation complexes with organic acids. Between them, ionic calcium, as measured by an ion selective electrode, was about 50 per cent of the total calcium. The remainder of the soluble calcium was present as complexes with citric and malic acids. Iron is transported mainly as Fe citrate, zinc can also be transported as a histidine complex, and Zn, Cu, Mn and Ni can be transported as
nicotianamine complexes. Nitrogen is mostly present in the xylem in its inorganic forms (NO3, NH4), although amino acids and amides have been also observed (Peuke, 2010). The proportions of the various N fractions in the xylem sap depend on the form of N supply (NO3, NH4) and the major site of nitrate reduction (roots or shoots). Except at very high external NH4 supply, usually the concentration of NH4 in the xylem is very low (Van Beusichem et al., 1988) and much less then nitrate anions.
Similarly, phosphate and sulphate are the dominant forms of P and S in the xylem. Besides all other elements, potassium is adsorbed and transported as separate ion. In apple species, high concentra- tions of sugars may also occur and sugars may account for about 15% of the total organic carbon in the xylem sap. Phytohormones are a normal constituent of xylem sap, particularly cytokinin which are mainly synthesized in the roots. The special attention was paid about the concentration of abscisic acid (ABA) in xylem sap, as a possible chemical signal to the shoot of root water status.
3.6 Phloem loading
Vascular system in plants allows the cycling of nutrient within the plants. By this way, nutrients are loaded in the stem and the leaves and then shared between growing organs (shoots, fruits and root).
This process of tissue supply, named as a phloem loading, after salutes processing are more saturated even up to the 15-25% of dry matter, having a higher pH (7–8). This increase of concentration for all nutrients, with exception of Ca, is usually being several times greater in phloem exudate than in the xylem exudates. The main organic component of phloem sap is usually sucrose, which may participate up to 90% of the solids. Besides the sucrose, a high concentration of amino compounds is present in phloem sap (Peuke, 2010), where the amides of glutamine and asparagine represent 90% of this fraction. On the other hand, the concentrations of nitrate and ammonium are usually low (Van Beusichem et al., 1988). Organic acids such as citrate and malate are also present in the phloem sap, and, succinate concentrations may reach the same concentration as total amino-N. A whole range of other organic compounds are also found in phloem sap, for example secondary metabolites,
hormones, proteins and RNA (Turgeon and Wolf, 2009). Having in mind that the phloem long distance transport takes part in sieve tube elements, a part of phloem salutes is transported between cells through the symplast pathway across the plasomdesmata (“protein channels between cells”). By phloem sharing of digested nutrients between organs, it is reflected a whole status of tree fruits nutrition. The amount of supplied and transferred phloem content reflects an initial process of nutrient adsorption.
One of the most important things in apple nutrition understands a nutrient storage and nutrient distribution within plants. A crucial thing is to create a difference between nutrient movement and storage in perennial plant, just like it’s happened in apple tree, and nutrient turnover in one-year old greenish plants. In cereals, like corn, wheat, rye, etc., most of the adsorbed nutrients go in the aboveground parts and this amount reach up to 90%, where only 10% remain in roots. So, when nutrient reserves are depleted from the initial seed reserve, the growth of young sowed plants completely depends of the soil nutrient uptake, namely, plant goes heterotrophic nutrition. After the development of cereal’s third leave, an absolute plant’s supply depends on root activity and nutrient reserves in soil, usually stocked by autumn/winter fertilization (P, K) and spring nitrogen dressing.
Plant’s development and yield formation are entirely coupled to the amount of added fertilizers, or
nutrients which are present or exist in soil. After adsorption, nutrients mainly go up, making the mentioned distribution ratio between aboveground and root organs. New growth or a new start of greenish plants growth, is, in general, related to the external nutrient reserves, where some additional supply of nutrients, besides fertilization, is obtained by plant’s residue mineralization. So, concerning that a large portion of biomass of sowed plants has destiny as a plant’s residue, the obtained by mineralization of soil’s nutrients should not be underestimated or neglected.
On the contrary, perennial plants have “storage organs” with absorbed nutrients. This means that some plants parts or organs have an additional role in the nutrient transfer from root to the top.
Storage and nutrient transfer of the wooden parts of plants could be compared to the “swimming pool”. This “pool” keeps all adsorbed nutrients, managing their distribution and finally ruling their stocked amount. This means that this “pool” will release collected nutrients according to the needs of certain phenological phases in the plant. In example start of root growth, flowering phase, or start of shoots growth, or phase of the yield formation, needs some of the stored nutrients in different amounts and different ratios. It could be said that this is regulated by the driven metabolic pathway.
Generally, present hormones trigger the processes of certain biochemical syntheses and this will need some elemental support. Crucial is to note that each of these elements derive from these reserves stocked in wooden plant parts. However, this wooden tissue balances them, because this “swimming pool’’ should not be ever completely emptied. The loss of stored nutrients could be depicted as “water overflow above the firm edges of the pool”. This means that stocked nutrients must be collected to a certain level in wooden tissues, where only the excess of their amount will allow this movement from reserves to the points of plant’s growth or development of new tissues. In conclusion, the nutrient movement is governed by enough stored reserves, which support or block a new growth or building of new tissues.
In the meantime, the “pool” reserves are renewed via root adsorption, keeping an initial nutrient content at the level which enables plant’s perennial life, especially in the periods which is not favourable for nutrient supply like droughts, flooding or simply the food shortage. As a result, investigations on perennial plants (Neilsen et al., 1997, Nachtigall and Dechen, 2006, Zanotelli et al., 2013, Tagliavini and Zanotelly, 2015) gave us reliable data that most of the nutrients are stored in root, stem and branches. With70% of stored nutrients these organs can be declared as “storage organs”.
This indication, however, could induce a misleading or misunderstanding about the total content of some element in different plant organs. This is typical for nitrogen as the most mobile and also most investigated element in plant tissues. Analyses can give us a presence of N in apple leaf between 2,10- 3,07%, much higher than the content in roots (0,74-0,78%) and stem and branches (0,45-0,48%) as woody organs (Tagliavini and Zanotelly, 2015). By the calculation about the weight of the formed biomass has been done (d.m.), the total yield and stored N in woody organs are significantly higher, giving 19 kg/ha of total N in leaves and 36 kg/ha of N which is placed in wooden parts.
This role of stored nutrients induces a term of remobilization, which means their reabsorption from storage organs. This is a mechanism for retaining and conserving nutrients in perennial plants, when remobilization appears to be an important component of nutrient use and efficiency. This is especially exposed when it comprises a seasonal nutrient cycling. Therefore, remobilization appears periodically and usually is related to the season’s changes. It is important to emphasize that early growth of apple, or growth of any part of perennial plant, are completely provided by the remobilization process. This practically means that there is no direct/instant effect of applied nutrients by fertilizers (mineral fertilizer) on apple growth, what is happening in fertilization of e.g. cereals. The adsorb nutrients in apple therefore should pass two steps after their uptake, where the first one is a deposition (storage organs) and, after, second one is internal redistribution. So, the real effects of fertilization will be shown after initial consumption of stored nutrients. Further nutrients uptake in later phase of running vegetation from fertilizer or from soil will support the further organs growth (leaves, buds’
differentiation, fruit and shoot growth). This remobilization mechanism should be considered when planning a mineral fertilizer application for apple, because the nutrient should be in adequate quantity
in organs when bud differentiation occurred. This specific growing phase of apple generally passed hidden, and it is totally separated from yield formation which is at this part of vegetation in focus of growers’ interest.
4 Functions of minerals in apple nutrition
The functions of minerals in apple nutrition are numerous. Their role in plant tissues are based on deep physiological investigation. Most of them catalyse biochemical processes. They are generally coped as building material for tissues growth, except potassium which exists in plant tissues as free cation. Naturally, they significantly vary according to their content in tissues, ranging from their trace amounts up to the percentage participation in tissues. One thing is very important for these 17 macro and microelements what should never be forgotten or thrown away: they are of equal importance for the plant’s growth. Also, for plant metabolism they do not have an alternative. Consequently, they must be present in demanding amounts in tissues to conduct a functional plant metabolism.
Nutrients affect the plant’s growth, cell division, yield, yield quality, mineral composition, quantity and quality of syntheses of primary and secondary metabolites, disease resistance, etc., where the role of each nutrient are precisely described. However, nowadays, the present investigation related to nutrients tends to explore their mutual interaction on different complex parameters, like the nutrient effect on components of fruit coloration, nutrient effect on synthesis of different type of carbohydrates or vitamins, a synthesis of apple phytochemical and their health benefits, effects on nutrient
antoxidative activity, or on nutrient rootstock/scion interaction etc. But the great number of them is still focused on nutrient effects on yield levels and fruit quality. These investigations are trying to find parameters for the practical use of nutrients to reach the two main goals in apple production: quantity and (market) quality. Up to now, it is obtained that whole complex of conducted investigation which confirms the positive role of nutrients in investigated biosynthesis.
Nitrogen: Nitrogen used by apple trees for growth can be derived from fertilizers, from soil, or it can be remobilized as N within the trees themselves as previously described. A mineralization of soil organic matter gives a certain amount of soil’s N, but this is not a major contribution to the plant’s N nutrition. This give the relatively respective amount of available N which depends primarily on a soil type, the degree and intensity of mineralization, but concerning that the orchards are not set up at the richest soil with organic matter, the content of such released nitrogen never exceed 20-25 kg N/ha per year (Rish et al., 2019). In practice, orchards are mainly supplied by N with liquid or granular mineral fertilizers, but if not, they will live only upon these reserves in soil and stored N in plant. In spring, with vegetation start, N is used from overwinter stored reserves in perennial, woody tissues by remobilization, so the growth of new roots, leaves, flowers and shoots is fully dependent on these reserves (Titus and Kang, 1982, Millard and Neilsen, 1989). Further, subsequent growth of apple trees during the summer, concerns a new accumulation of N by the roots, but this amount has been
predominantly translocated in the leaves. This N stays in leaves before its withdrawal during senescence (Neilsen et al., 1997), and this withdrawal of N during autumn could also significantly enriched the “pool” of nitrogen for further remobilization. Also, during the autumn, up to the existing root activity before winter, N is also fulfilling woody tissues with N and after that, the adsorb N undergoes internal cycling (Ličina and Jakovljević, 1996).
Nitrogen is stored in perennial parts of apple trees over winter in roots as amino acids (Tromp, 1983), as proteins in the stem (Millard and Proe, 1991) and in bark (Titus and Kang 1982). In leaves, during the summer growth, N primarily participate in the building of Rubisco (Ribulose-1,5-bisphosphate carboxylase /oxygenase) (Titus and Kang 1982, Millard, 1996), the most present enzyme (40%) in leaves biomass (Teiz and Zeiger, 2010). Withdraw of N from leaves occurs during periods of
senescence, when N present in leaves (N in amino acids) has been transferred in this form to woody parts. This N also makes a significant N- pool for subsequent remobilization and use for growth in the spring It is believed that 50% of the total N is in soluble form (amino acids), but Titus and Kang (1982) argued that proteins are the main storage form of N woody parts, with free amino acids of secondary importance. Parallel to this explanation, nitrogen cycling from leaves to woody parts must also be concerned as transfer of N from leave’s compartments, what seems to be the key determination of such
nitrogen allocation. This means that structural nitrogen, like N built up in a cell wall, contributes little to nitrogen resorption, because it has low degradability during leaf senescence, while metabolic nitrogen, like N from enzymes, is largely resorbed (Yasumura et al., 2006).
In apple physiology, nitrogen is the main constituent of proteins, amino acids, nucleic acids,
chlorophyll, co-enzymes, phytohormones and secondary metabolites and therefore plays a major role in plant metabolic processes (Salisbory and Ross, 1992, Neilsen and Neilsen, 2003). Its requirements are higher than the requirement for any other nutrients. Nitrogen support’s growth of new tissues formation, such as a developing leaves, shoots and fruits (Neilsen and Neilsen, 2003). It is well known that the deficiencies of N induce poor growth, low yield and leads to small fruits. Adequate supply of nitrogen should be during bud differentiation or otherwise its number will be significantly reduced.
For better understanding, the nitrogen movement in trees if various tree parts are analysed. The Nitrogen content tends to be high in growing shoot tips, growing leaves and young fruits. However, high levels of nitrogen supply induce vigour growth, development of water shoots, induce fruit drop and increased physiological disorder in fruits (Hewitt and Smith, 1975, Faust, 1989). Excess of nitrogen also reduced fruit coloration and aggravate fruit maintenance over the time. So, concerning all aspect of N importance for plants nutrition, the availability of N to roots is therefore a decisive factor for apple growth.
Phosphorus: Requirement of phosphorus in apple growth is relatively small compared to other nutrients. Phosphorus stays as a key factor in compounds which provide energy transfer, also, phosphorus is a constituent of nucleic acids and, therefore, it is mostly required in plants during the stages of meristematic activity. This particularly concerns the start of growth caused by the seasonal changes or seedling’s planting. A development of a new root system, flower formation, pollination and fruit setting are the periods when P plays a crucial role in plants. Therefore, the needs for phosphorus are generally exposed early in the season. Phosphorus deficiencies are harmful for the wide range of metabolic processes by delaying plant growth, causing poor root growth, and reducing a fruit size and quality of the yield (Marchner, 2002).
In plant supply with phosphorus, the main problem is its low content and its low mobility in soil. This is especially exposed for grown fruits, because apple root system is placed at deeper layers concerning surfaces one. However, by the use of fertigation system in orchards’ nutrient supply, most of the problems of feeding fruits with phosphorous has been overcome. Phosphorous in nutrient solution can be applied to the root zone more often at different phenological phases to support growth of plants or to affect the development of some tissues like root tissues during elongation, flower tissues
development and fruit setting. This fertilization technique completely erased the problem of phosphorus inactivation after its reservoir application during the soil preparation for orchards’
planting. This has been usually conducted using huge amounts of phosphorus fertilizer which is subjected to the complex process of P immobilization in root’s feeding layer. Accepted forms of phosphorus for plants are H2PO4- ion or HPO4-2 ion, which is affected by soil’s pH. The first one is available on acid soil, and second one on neutral and alkaline soils.
Potassium: Potassium is of a great importance for apple production. The present amount of this element in leaves and fruits are close to the level of nitrogen. In leaves, where the intensity of metabolic processes is the most exposed, potassium is the most abundant cation between others mineral nutrients (Neilsen and Neilsen, 2003). This element is responsible for photosynthesis, protein synthesis, enzyme activation (about 60 catalytic activities), osmoregulation, stomata movement and cell extension. Secondary metabolites are also very dependent on potassium level in cytoplasm. Its mobility and its fluctuation are driven by metabolic processes, where K transfer in plant is usually tied up to phloem and xylem vessels, but significant transport take place between cells (symplast pathway).
Apple uptake of potassium by roots usually depends on its quantity in soil, concerning that this element could be taken up opposite to the concentration gradient. In root tissue its concentration can exceed a hundred times of its soil’s concentration, but still uptake occurred. The supply of potassium
to apples trees primary depends on fertilizer application, considering that pH of soil solutions have no effect on its adsorption by plants. Otherwise the type and character of soil mostly affects its level in the ground and afterwards in plants. Its application is evidenced by increased fruit size, increased sugars and improved fruit colour (Rambola et al., 2000), thanks to its high osmotic potential which possess this free cation which is not tied up to any constituent compound in cytoplasm. Potassium application is particularly effective if it’s happened during midstages of fruit development or, in soil rich in clay, at the end of growing season (Tagliavini and Marangoni, 2002). Its single use is usually avoided, because of potential adverse effect on Ca nutrition. Now, by using fertigation, K can be applied more often to the root zone and at the certain fruit developing stage.
Calcium: One of the major concerns for an apple grower is calcium, dispute that this element usually saturated soil’s adsorptive complex to a large extent (60-80% CEC). In soil poorly provided with Ca, all problems related to the lack of Ca are about the bed fruit storage life: fruit quality is threatened by bitter pit, Jonatan spot or scald appearance. This is because calcium serves important functions within the plants, including the cellular behaviour and maintenance of cell division, cell integrity and
membrane permeability (Mengel and Kirkby, 1982). However, one of the bad Ca futures is a low mobility in plants tissues, where no transfer between organs occurred. So, feeding apples with this element generally concern its foliar application on fruits during developing period. This is a direct care of the fruits and not leaves, and the goal of this measure is to make an increase of Ca concentration in outer fruit layer.
Magnesium: Magnesium is taken up by fruit trees in lower quantities then Ca. It is worth to know, that magnesium adsorption can be reduced by competing cations such as K, Ca or NH4+, especially when heavy K fertilizer use emphasize this antagonism in fruit production. Its role in plants is mostly associated with the chlorophyll formation, as an ion which possess a central place between four N terminated tetrapyrrole rings, but only 10-15%, or maximum 20% of its total content in plants belongs to this Mg fraction. The majority of Mg in tissues serves other not less important biochemical
functions in plants, like enzymes activation involved in phosphorylation, activation of RUBISCO and protein synthesis (Mangel and Kirkby, 1982). In the concept of contemporary apple nutrition, Mg plays a significant role as regularly applied element through fertigation systems especially in orchards planted on soils with low clay content.
Sulphur: In apple trees, sulphur is required approximately at the same quantity as phosphorous. Like in the other plants, this macroelement is a structural part of sulphur-containing amino acids (cysteine, cystin and methionine), proteins and co-enzymes (Neilsen and Neilsen, 2003). In trees, sulphur also makes a wide range of stabile chemical complexes which provide metals binding to organic
compounds, mostly with a catalytic enzyme character.
Iron: Apple growers very often fight against iron chlorosis, as a yellowing of young leaves caused by iron deficiency. In severe cases, the entire leaf turns yellow and it is common for an individual branch or one half of a tree to be chlorotic while the remainder of the tree appears normal. In some areas the entire orchards may be affected, while in others only the most susceptible plants show deficiency symptoms. These yellow leaves indicate a lack of chlorophyll and this reduction in chlorophyll content during the growing season can reduce plant growth and vigour. In addition, chlorotic plants are less productive, producing smaller fruits, while threatened orchards are without adequate yield. In severe cases if iron chlorosis persists over years, individual branches or the entire plant may die. The causes of iron chlorosis are complex and not completely clear, but everything about this deficiency is affected by reduced iron availability in soil. There is also some scientific indication that Fe problem also exists at the tissues level, where its Fe2++ physiological activities failed. Usually, iron chlorosis is related to the high pH of the soil (pH > 7.0) and soils poor aeration and excess of moisture (lime-induced chlorosis). Such soils typically have plenty of iron, but soil’s high pH and carbonates causes chemical reactions that make the iron unavailable to plants. Such iron will be tied up indefinitely unless soil conditions change.
Iron is a one of the rare micronutrients with has multiple biochemical functions in plants. It is responsible for chlorophyll synthesis, and whole photosynthetic complex in leaves are built on this element. Also, huge number of catalytic reactions in plans relates to iron participation like a crucial process in plant cell: respiration, cell division, cell growth, etc. This gave a special attention to Fe in apple production, where some of the invented Fe fertilizers can successfully overcome the problem of its low availability in soil (Fe-chelates).
Manganese: Manganese in plant production belongs to three micronutrients which are responsible for yield size. Its participation in photosynthesis and nitrogen and carbohydrate metabolism seems to be crucial for this characterization. It is generally considered to be rather immobile in plants, but its increased concentration was dominantly observed in young growing tissues. The lack of Mn in leaves is easy to recognize, because specific “mosaic chlorosis” appears (intercostals parts are yellow, while leave nerves stay green). In Tyrol (Italy), as a well-known apple production area in the world, its foliar application sustains in practice for years.
Zinc: In tree plants, zinc is needed in small amounts, however, this microelement participates in huge number of enzymatic reactions. A special attention has been paid to its role in the production of the growing hormone auxin, which is responsible for the entire growth process. Proper cell division will terminate the shortening of internodes and formation of small leaves what is observed with zinc inadequate supply. The mobility of zinc in the tree depends on several factors. If the plants are with adequate Zn supply, zinc moves readily from old leaves to developing ones, otherwise, little zinc moves out of the old leaves if deficiency exists. Compare to the other fruits, e.g. stone fruits, apple is not so particularly sensitive to the zinc deficiency, but apple fertigation practice today frequently include this element in nutrition (Zn-chelates), while its foliar spraying in some apple growing regions became a routine.
Boron: Between micronutrients, boron trigs a great attention in plant nutrition concerning that it can be toxic. This is especially a big problem in orchards and vineyards planted in deserts. Due to the lack of the electrical charge of the B-acid molecule, B excess is easy to rinse with water, what start to be a practice from vegetation to the vegetation in some growing area. Only the application of this measure in such potentially B toxic regions provide fruit or grape production.
In plants, boron is needed for several metabolic processes, particularly in the activities of meristematic tissues. Cell division, supported by the uracil bases synthesis, makes the boron functions vital in the development of shoot’s and tips tissues. It also plays a role in protein synthesis and metabolism of plant hormones, coupled with its capability to facilitate sugars transport, promote the importance of this non-metals for apple growth and yield quality. Its transport in plants has been relived as problem by its movement almost exclusively with the transpiration stream in the xylem. However, its mobility between organs usually is not exposed.
Copper: Despite that copper belongs to the element’s which provides a high yield, a very small quantity of these elements is present in fruit trees. More than half of this copper in trees is in the chloroplasts, which participate in photosynthetic reactions. It is also found in other enzymes involved with protein and carbohydrate metabolism. Relocation of Cu in tissues is conditioned by its adequate supply. If the deficiency exists, it becomes immobile what is shown first at young leaves.
Molybdenum: Molybdenum is a component of two enzymes which deals with nitrogen metabolism, and their activities are more vital for annual plants, so its appearance and importance for apple production is minor or yet not define.
